With the recent increase of data traffic, high-speed, large-capacity communications have become indispensable, so that the construction of large-capacity networks using communication technologies such as DWDM (Dense Wavelength Division Multiplexing) is being advanced. Also, in anticipation of the realization of even larger-capacity data transmission in the future, the construction of next-generation photonic networks capable of dynamic wavelength switching and routing is hoped for. As a solution to the implementation of such networks, variable-wavelength light source device has been attracting attention which is capable of outputting a desired wavelength.
FIG. 17 illustrates the configuration of a variable-wavelength light source device, or more specifically, a temperature tunable-type variable-wavelength light source device 30 using an 8-channel DFB (Distributed Feedback)-LD (Laser Diode) array.
The variable-wavelength light source device 30 comprises DFB-LDs 31-1 to 31-8, an optical coupler 32, and an SOA (Semiconductor Optical Amplifier) 33. The wavelength output end of each of the DFB-LDs 31-1 to 31-8 and the wavelength input end of the SOA 33 are coupled by the optical coupler 32 such that the wavelength output from one of the DFB-LDs 31-1 to 31-8 is amplified by the SOA 33 and output to the outside of the device.
FIG. 18 illustrates the operation of the variable-wavelength light source device 30, wherein the horizontal axis indicates wavelength and the vertical axis indicates optical power. Specifically, FIG. 18 schematically depicts wavelength ranges H1 to H8 respectively covered by the eight DFB-LDs 31-1 to 31-8 of the variable-wavelength light source device 30.
The DFB-LD 31-1 outputs a desired wavelength variable within the wavelength range H1, the DFB-LD 31-2 outputs a desired wavelength variable within the wavelength range H2, and so on. Thus, the DFB-LD 31-8 outputs a desired wavelength variable within the wavelength range H8.
To cause the variable-wavelength light source device 30 to output a wavelength λa, indicated in FIG. 18, as a variable wavelength output, for example, first, the DFB-LD 31-1 with the wavelength range H1 including the wavelength λa is selected from among the DFB-LDs 31-1 to 31-8. Then, the laser temperature is varied and set to a laser temperature Ta for outputting the wavelength λa.
In this manner, when a desired wavelength is to be output from the variable-wavelength light source device, the DFB-LD to which is allocated the wavelength range including the desired wavelength is selected, and the laser temperature is adjusted so that the desired wavelength may be output from the selected DFB-LD.
As stated above, the variable-wavelength light source device is constructed by monolithically integrating a DFB laser array, an optical coupler and an optical amplifier on a single chip and permits one of multiple wavelengths to be output therefrom by the selection of a DFB laser and the temperature control. The variable-wavelength light source device can replace multiple light sources that are needed for respective wavelengths in conventional WDM systems, making it possible to reduce costs of WDM systems and simplify the maintenance and management of the systems.
As conventional techniques related to the variable-wavelength light source, a technique has been proposed wherein, at the time of switching wavelengths, an electric current is injected into a semiconductor laser, and after the temperature becomes stable, the power supply to the laser diode (LD) is controlled (e.g., Japanese Laid-open Patent Publication No. 2005-64300).
The temperature tunable-type light source like the above one has been mainly used until now as the variable-wavelength light source device. Since this type of light source device has a construction such that the refractive index in the laser diode active layer is varied by the application of heat, however, the tuning speed is slow in principle and of the order of milliseconds (ms). Thus, the temperature tunable type is unsuitable for high-speed applications such as high-speed wavelength switching and high-speed wavelength routing. In recent years, therefore, current injection-type variable-wavelength light source devices capable of high-speed tuning on the order of nanoseconds (ns) have been developed.
FIG. 19 illustrates the structure of a current injection-type DFB-LD. A gain control layer (act) 301 and a tuning layer (tuning) 302 are alternately arranged on a waveguide 300 (the gain control layer 301 is a layer that contributes to the emission of light, and the tuning layer 302 is a layer for wavelength setting), and electrodes 303 are arranged atop so as to correspond in position to the alternately arranged layers. Also, a grating (diffraction grating) 304 is formed beneath the gain control layer 301 and the tuning layer 302.
Thus, in the current injection-type variable-wavelength light source device, the wavelength control layer is provided in the waveguide separately from the gain control layer. The set wavelength is controlled by injecting an activation current into the gain control layer to control the power of the laser diode and also by injecting a tuning current into the tuning layer to vary the carrier density and thereby change the refractive index. By virtue of the aforementioned structure and control, high-speed tuning of the nanosecond (ns) order can be achieved.
In the current injection-type variable-wavelength light source device, however, noise superimposed on the tuning current directly appears in the form of wavelength variation, causing spectral linewidth broadening. Thus, if an optical signal with a broadened wavelength spectrum is transmitted during the operation, a problem arises in that conspicuous waveform degradation occurs.
FIG. 20 illustrates the relationship between current noise and transmission penalty, wherein the horizontal axis indicates noise frequency (kHz) and the vertical axis indicates transmission penalty (dB) (transmission penalty: a measure of the rate of discrimination error in the received signal attributable to transmission). FIG. 20 reveals that the transmission-induced degradation worsens with increase in the noise frequency of the tuning current.
In order to improve transmission characteristics, therefore, high-frequency noise needs to be suppressed. In a conventional method for suppressing high-frequency noise superimposed on the tuning current, a bypass capacitor is arranged in the vicinity of a tuning current generating section, for example, to allow the cutoff frequency to be set to a lower frequency, thereby suppressing the high-frequency noise. With this method, however, the tuning speed lowers because of the time constant of the capacitor, giving rise to a problem that the intended high-speed tuning (high-speed wavelength switching) fails to be attained.